I'm looking at a Carbon X1 (3rd Gen Broadwell refresh) with 8gb RAM (maxed) but having trouble deciding on a processor. Assumed I wanted/needed the i7, but the upgrade is $350 -- which seems way overpriced.

In real world Adobe CS use, do you think I'll feel a big performance compromise with the i5? This will be my "travel machine" primarily for writing and presentations, but several times a month I do photo or video shoots. So media transfers plus lots of Lightroom, some Photoshop and occasional light use of Premiere for HD and 4K footage. (I have a desktop for serious editing.)

Hate being fleeced by Lenovo, but should I get the i7-5600U anyway?(Still 2 cores hyper-threaded, but i7 has 33% more cache, 10-15% higher clock speeds and Intel graphics at 950Mhz instead of 900Mhz.) Do you think these differences would be noticeable for the kind of media work I described?

Thanks in advance for any thoughts or real world experience using CS on an i5 laptop..

Well I do not think you will be satisfied with either for Premiere for HD in a timeline and absolutely no way would you be able to handle any 4K. I am delighted with my laptop with a quad core i7-4700HQ and the nVidia GTX 765M. If as you said just media transfers then it will be fine. Just make sure that Lenovo did not load it with Superfish

On top of that, you would benefit massively from having a decent hardware GPU (NVidia) to support 4K real-time playback.

If you think CC_Merchant and I are being hardware snobs, try a test with your desktop set up with only 8GB of RAM, limit your bios settings to 4 cores and no hyuperthreading (similar to 2 cores w/ hyperthreading), and set Adobe to only use software rendering (no GPU) and see if the performance will even "get you by" when traveling.

Your budget does not need to go higher, but you will need to be willing to travel with another 1 or 1.5 lbs to get a decent 14" travel laptop for Premiere Pro.